Although some of the pieces are still missing, the available pieces of the mosaic begin to fall in place and a coherent picture is emerging. In this picture, the Laki-Lorand factor (Facdor XIII) and tissue transglutaminase occupy a central position. This enzyme is responsible for carrying out the "masking" process by attaching proteins to the histocompatibility antigen to the tumor antigen. Keeping these two types of antigen separated in space or in time, their joint role as signals for the T-lymphocytes is prevented.